Many greenhouse gases absorb infrared radiation in a 4-12 μm wavelength region, as shown in FIG. 1, which is a graph of the absorption of infrared radiation by CO2 gas as a function of wavelength. Present non-dispersive sensors rely upon an infrared (IR) source like a filament bulb or an LED/laser together with filters and a light pipe to carry out the analysis. Present non-dispersive sensors use one of two approaches for determining the concentration of gas analyzed.
FIG. 2 is a diagram illustrating a conventional gas sensor system 200 using a first approach in which the system 200 determines a ratio of a test system 202 against a reference system 212. The test system 202 comprises a light pipe 203, an emitter 204. and a detector 206. The light pipe 203 is open at both ends to allow a gas under test to flow through the light pipe 203. The emitter 204 and the detector 206 are disposed at opposite ends of the light pipe 203 so that light emitted by the emitter 204 propagates through the gas in the light pipe 203 and is partially absorbed if the light emitted by the emitter 204 is at an absorption frequency of the gas that is being tested for. The reference system 212 comprises a light pipe 213, an emitter 214, and a detector 216. The light pipe 213 is sealed and typically contains a vacuum or an inert gas. The emitter 214 and the detector 216 are disposed at opposite ends of the light pipe 213. The conventional gas sensor system 200 detects the received signals at the detectors 206 and 216, and compares the test signal in the test system 202 to a reference beam in the reference system 212 that is at a non-absorbing infrared wavelength.
The conventional gas sensor system 200 requires duplication of hardware (e.g., two emitters, two detectors, and two light pipes), and an optical sensing path in the light pipe 203 that must be keep clean.
FIG. 3 is a diagram illustrating a conventional gas sensor system 300 using a second approach in which the system 300 determines a ratio of two infrared signals in a common path. The conventional gas sensor system 300 comprises a first emitter 304 that generates an infrared beam having a first frequency that is directed towards a mirror 303 and a first detector 306 for detecting an infrared beam having the first frequency that is reflected from the mirror 303. The conventional gas sensor system 300 comprises a second emitter 314 that generates an infrared beam having a second frequency that is directed towards the mirror 303 and a second detector 316 for detecting an infrared beam having the second frequency that is reflected from the mirror 303.
The conventional gas sensor system 300 detects the received signals at the detectors 306 and 316, and compares the two detected signals to each other. By selecting the second frequency to be at a frequency that the gas under test does not absorb, the ratio of the compared signals is indicative of whether the gas under test is the gas that is being tested for.
The conventional gas sensor system 300 is prone to error because of unknown contaminants in the light path that have unknown frequency absorption characteristics. Further, a second emitter and a second detector are required.
What is needed is a system and method for detecting gases with less hardware and less errors due to contamination.